Voltage-controlled capacitance elements, or varactors, are useful in oscillator circuits that are commonly used in communication devices, as well as in many other applications. Resonant tank circuits are often used for radio frequency oscillator circuits because of their high-resonance frequency of operation, low phase and amplitude noise, and low power consumption. In order to tune such a circuit, capacitance is typically controlled by an external circuit to vary the frequency of resonance. A large capacitance control variance range is often necessary to compensate for manufacturing variances of the individual components, as well as to provide a means to cover a wide tuning frequency range in many applications. This is often accomplished by means of coarse tuning switched capacitors applied to the tank circuit and a fine-tuning voltage-controlled capacitance, or varactor, generally driven by an analog control feedback loop such as a phase-locked loop system.
In such a control feedback system, it is desirable to control the voltage-to-frequency gain constant across the tuning range. This implies that the voltage-to-capacitance transfer function be linear. This also requires that the fine tune capacitance range be a constant percentage of the total tank capacitance. The fine-tuning capacitance range must increase when switching more coarse tuning capacitance onto the tank circuit. In voltage-controlled oscillators, this gain factor is referred to as Kvco, or the gain of the voltage-controlled oscillator.
With regard to the varactor element of the oscillator circuit, the two primary factors that contribute to phase noise are (i) the quality factor, Q, of the varactor element and (ii) the non-linear varactor capacitance-versus-voltage transfer characteristics that cause excessively high voltage-controlled oscillator gain.
One type of varactor element, the metal-oxide-semiconductor (MOS) varactor element, has been shown to have advantageous quality factor characteristics over the more common junction varactor elements. However, the strongly non-linear capacitance-voltage (CV) transfer characteristic, coupled with a very limited tuning range, has limited the usefulness of MOS varactor elements. In addition, some MOS varactor elements have a non-monotonic CV transfer characteristic just outside the upper end of their limited tuning range.
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